Chapter 8: Weathering, Soil, and Mass Movement

lahar

-Mudflows on the slopes of volcanoes that result when unstable layers of ash and debris become saturated and flow downslope, usually following stream channels.

sheet erosion

-Peeling off thin layers of soil from the land surface; accomplished primarily by wind and water -the process by which water flows over a layer of soil and removes the topsoil

external process

-Process such as weathering, mass wasting, or erosion that is powered by the Sun and transforms solid rock into sediment - seeks to move debris to lower elevations

loam

-Rich, fertile soil that is made up of about equal parts of clay, sand, and silt.

Solum (true soil)

-The O, A, and B horizons in a soil profile. Living roots and other plant and animal life are largely confined to this zone.

leaching

-The depletion of soluble materials from the upper soil by downward-percolating water

weathering

-The disintegration and decomposition of rock at or near Earth's surface.

mass movement (mass wasting)

-The downslope movement of rock, regolith, and soil under the direct influence of gravity.

Earhtflow

-The downslope movement of water-saturated, clayrich sediment. Most characteristic of humid regions. - forms often on hillsides during times of heavy prepcipitation -When water saturates the soil and regolith on a hillside, the material may break away, leaving a scar on the slope and forming a tongue- or teardrop-shaped mass that flows downslope - moves at slower rates because it is very vsicous

slump

-The downward slipping of a mass of rock or unconsolidated material moving as a unit along a curved surface. -occurs most commonly due to oversteepening of slopes

erosion

-The incorporation and transportation of material by a mobile agent, such as water, wind, or ice.

regolith

-The layer of rock and mineral fragments that nearly everywhere covers Earth's surface

frost wedging

-The mechanical breakup of rock caused by the expansion of freezing water in cracks and crevices.

mechanical weathering

-The physical disintegration of rock, resulting in smaller fragments.

chemical weathering

-The process that breaks down rock through chemical changes -The processes by which the internal structure of a mineral is altered by the removal and/or addition of elements

rockslide

-The rapid slide of a mass of rock downslope along planes of weakness. - when material is mostly soil/regolith debris is used - Such events are among the fastest and most destructive mass movements.

creep

-The slow downhill movement of soil and regolith.

differential weathering

-The variation in the rate and degree of weathering caused by such factors as mineral makeup, degree of jointing, and climate

eluviation

-The washing out of fine soil components from the horizon by downward-percolating water.

rock avalanches

-Very rapid downslope movement of rock and debris. These rapid movements may be aided by a layer of air trapped beneath the debris, and they have been known to reach speeds of over 200 kilometers (125 miles) per hour

peds

Clumps that give soil a particular structure - ex. Platy, Prismatic, Blocky, and Spheroidal

spheroidal weathering

-Any weathering process that tends to produce a spherical shape from an initially blocky shape.

B horizon (subsoil)

- zone where much of the material removed from the E horizon by eluviation is deposited. Thus, the B horizon is often referred to as the zone of accumulation. The accumulation of the fine clay particles enhances water retention in the subsoil. In extreme cases, clay accumulation can form a very compact and impermeable layer called hardpan.

soil

-A combination of mineral and organic matter, water, and air; the portion of the regolith that supports plant growth. - 25% air, 25% water, 45% mineral matter, 5% organic matter

exfoliation domes

-A large, dome-shaped structure, usually composed of granite, formed by sheeting. -Rounded mountain tops formed by the peeling off of large sheets of rock along curved surfaces

soil horizons

-A layer in a soil profile

sheeting

-A mechanical weathering process characterized by the splitting-off of slablike sheets of rock - occurs when large masses of igenous rocks are exposed by erosion - lack of confining pressure due to overlying rock makes fracture

slides

-A movement common to mass-wasting processes in which the material moving downslope remains fairly coherent and moves along a well-defined surface.

internal processes

-A process such as mountain building or volcanism that derives its energy from Earth's interior and elevates Earth's surface.

debris flow

-A relatively rapid type of mass wasting that involves a flow of soil and regolith containing a large amount of water. - is called mud ____, when most of the material is faine grained. - happens when a cloudburst or rapidly melting mountain snows create a sudden flood in a semiarid region, large quantities of soil and regolith are washed into nearby stream channels because there is usually little vegetation to anchor the surface material. The end product is a flowing tongue of well-mixed mud, soil, rock, and water

solifluction

-A slow, downslope flow of water-saturated materials common to permafrost areas, can also occur in dense clay hardpan areas, or to an area with an impermeable bedrock

Soil taxonomy

-A soil classification system consisting of six hierarchical categories based on observable soil characteristics. The system recognizes 12 soil orders. - broadest classification= order, most specific=series

flow

-A type of movement common to mass-wasting processes in which water-saturated material moves downslope as a viscous fluid.

fall

-A type of movement common to mass-wasting processes that refers to the free falling of detached individual pieces of any size. -are common on slopes that are too steep for loose material to remain on the surface. Many are the result when freeze-thaw cycles and/or the action of plant roots loosen rock and then gravity takes over - when rock does this, this is how talus slopes are built

soil profile

-A vertical section through a soil, showing its succession of horizons and the underlying parent material

O soil horizon

-a soil horizon that consists largely of organic material. This is in contrast to the layers beneath it, which consist mainly of mineral matter. -The upper portion is primarily plant litter, such as loose leaves and other organic debris that is still recognizable. -By contrast, the lower portion of the O horizon is made up of partly decomposed organic matter (humus) in which plant structures can no longer be identified. In addition to plants, this zone is teeming with microscopic life, including bacteria, fungi, algae, and insects. All these organisms contribute oxygen, carbon dioxide, and organic acids to the developing soil

E horizon

-is a light-colored layer that contains little organic material. As water percolates downward through this zone, finer particles are carried away - zone of leaching and eluviation

A horizon

-is largely mineral matter, yet biological activity is high and humus is generally present—up to 30 percent in some instances

transported sediment

-soil that does not match the rock under it.. soil that was transported in from another location

Rill erosion

-the removing of thin layers of soil as little rivulets of running water gather and cut small channels in the soil

parent material

-the source of the weathered mineral matter from which soils develop

C horizon

-zone that is characterized by partially altered parent material. Whereas the other horizons bear little resemblance to the parent material, it is easily identifiable in this horizon. -Although this material is undergoing changes that will eventually transform it into soil, it has not yet crossed the threshold that separates regolith from soil.

trigger

An event that initiates a mass movement process -Among the common factors that initiate mass-wasting processes are saturation of material with water, oversteepening of slopes, removal of anchoring vegetation, and ground vibrations from earthquakes.

permafrost

Any permanently frozen subsoil - usually found in subarctic and arctic regions

-An earthquake and its aftershocks can dislodge enormous volumes of rock and unconsolidated material which can start a mass movement

How can Earthquake trigger mass movement

-Plant roots in search of minerals and water grow into fractures, and as the roots grow, they wedge the rock apart. - Burrowing animals further break down the rock by moving fresh material to the surface, where physical and chemical processes can more effectively attack it. - Decaying organisms also produce acids, which contribute to chemical weathering. Where rock has been blasted in search of minerals or for road construction, the impact of humans is particularly noticeable -Some bacteria extract compounds from minerals in rcok

How can biological activity weather rocks

- plants and animals provide organic matter to the soil ( mainly from plants), this affects soil fertility - decay of organism produces organic acids which speed up the weahtering process. they can also aid in water-retention - microorganisms decay plants and animals and produce hummus - they also aid in soil fertility because they can fix nitrogen - earthworms mix and enrich soils, they also form holes which allows air and water to pass through

How do plants and animals affect soil formation?

-Acids such as carbonic acid readily decompose many rocks and produce certain products that are water soluble. - ex. calcite and feldspar

How does carbonic acid perform chemical weathering

-Climate is the most influential control of soil formation. Just as temperature and precipitation are the climatic elements that influence people the most, so too are they the elements that exert the strongest impact on soil formation. -Variations in temperature and precipitation determine whether chemical or mechanical weathering predominates. They also greatly influence the rate and depth of weathering. For instance, a hot, wet climate may produce a thick layer of chemically weathered soil in the same amount of time that a cold, dry climate produces a thin mantle of mechanically weathered debris. -Also, the amount of precipitation influences the degree to which various materials are removed (leached) from the soil, thereby affecting soil fertility. -Finally, climatic conditions are important factors controlling the type of plant and animal life present

How does climate affect soil formation?

- Potassium feldspar in granite reacts with water and carbonic acid to produce clay minerals, K+ions, biocarbonate ions and dissolved silica which eventually solidifies to form chert or flint - quartz is highly resistant to chemical weathering

How does granite weather? What are the products of the reaction

-The nature of the parent material influences soils in two ways. First, the type of parent material affects the rate of weathering and thus the rate of soil formation. (Consider the weathering rates of granite versus limestone.) Also, because sediments are already partly weathered and provide more surface area for chemical weathering, soil development on such material usually progresses more rapidly. -Second, the chemical makeup of the parent material affects the soil's fertility. This influences the character of the natural vegetation the soil can support

How does parent material influence soil formation?

-Plants protect against erosion and contribute to the stability of slopes because their root systems bind soil and regolith together. In addition, plants shield the soil surface from the erosional effects of raindrop impact (see Figure 4.20). Where plants are lacking, mass wasting is enhanced, especially if slopes are steep and water is plentiful.

How does removal of vegetation play a role in mass wasting?

- Rocky shorelines and arid regions are common settings for this process. It begins when sea spray from breaking waves or salty groundwater penetrates crevices and pore spaces in rock. -As this water evaporates, salt crystals form. As these crystals gradually grow larger, they weaken the rock by pushing apart the surrounding grains or enlarging tiny cracks.

How does salt crystal growth weather rocks?

-It occurs in a zone above the permafrost called the active layer, which thaws in summer and refreezes in winter. -During summer, water is unable to percolate into the impervious permafrost layer below. - As a result, the active layer becomes saturated and slowly flows. The process can occur on slopes as gentle as 2-3 degrees. -Where there is a well-developed mat of vegetation, a solifluction sheet may move in a series of well-defined lobes or overriding folds

How does solifluction occur in permafrost areas?

-. The nature of soil is strongly influenced by the length of time that processes have been operating. If weathering has been going on for a comparatively short time, the parent material strongly influences the characteristics of the soil. -As weathering processes continue, the influence of parent material on soil is overshadowed by the other soil-forming factors, especially climate. -The amount of time required for various soils to evolve cannot be specified because the soil-forming processes act at varying rates under different circumstances. However, as a rule, the longer a soil has been forming, the thicker it becomes and the less it resembles the parent material.

How does time affect soil formation?

- On steep slopes, soils are often poorly developed. In such situations, little water can soak in, and as a result, soil moisture may be insufficient for vigorous plant growth. Furthermore, because of accelerated erosion on steep slopes, the soils are thin or nonexistent - In contrast, waterlogged soils in poorly drained bottomlands have a much different character. Such soils are usually thick and dark. The dark color results from the large quantity of organic matter that accumulates because saturated conditions retard the decay of vegetation. -The optimum terrain for soil development is a flat-to-undulating upland surface. This terrain experiences good drainage, minimum erosion, and sufficient infiltration of water into the soil. -Slope orientation, the direction a slope is facing, also is significant. In the midlatitudes of the Northern Hemisphere, a south-facing slope receives a great deal more sunlight than does a north-facing slope

How does topography influence soil formation

-Water is by far the most important agent of chemical weathering. Although pure water is nonreactive, a small amount of dissolved material is generally all that is needed to activate it. Oxygen dissolved in water will oxidize some materials

How does water perform chemical weathering

-When the pores in sediment become filled with water, the cohesion among particles is destroyed, and they can slide past one another with relative ease. -Thus, saturation reduces the internal resistance of materials, which are then easily set in motion by the force of gravity. -Water also adds considerable weight to a mass of material. The added weight in itself may be enough to cause the material to slide or flow downslope.

How does water play a role in mass movement

examining Bowen's reaction series (see Figure 3.12, page 70), you can see that olivine crystallizes first and is therefore the least resistant to chemical weathering, whereas quartz, which crystallizes last, is the most resistant.

In what order are silicate minerals resistant to weathering?

soil texture

The relative proportions of clay, silt, and sand in a soil. A soil's texture strongly influences its ability to retain and transmit water and air.

1. Type of material 2. the kind of motion 3. velocity of the movement

What 3 criteria are used to classify a mass movement

frost wedging, salt crystal growth, sheeting, biological activity

What are 4 types of physical weathering?

-Quartz Quartz grains Silica - Feldspars Clay minerals Silica, K1, Na1, Ca21 -Amphibole Clay minerals Silica, Ca21, Mg21 Iron oxides Olivine Iron oxides Silica, Mg2+

What are some of the products of weathering of silicate minerals?

1.Alfisols Moderately weathered soils that form under boreal forests or broadleaf deciduous forests, rich in iron and aluminum. Clay particles accumulate in a subsurface layer in response to leaching in moist environments. Fertile, productive soils, because they are neither too wet nor too dry. 2. Andisols Young soils in which the parent material is volcanic ash and cinders, deposited by recent volcanic activity. 3.Aridosols Soils that develop in dry places; insufficient water to remove soluble minerals; may have an accumulation of calcium carbonate, gypsum, or salt in subsoil; low organic content. 4. Entisols Young soils having limited development and exhibit properties of the parent material. Productivity ranges from very high for some that form on recent river deposits to very low for those that form on shifting sand or rocky slopes. 5. Gelisols Young soils with little profile development that occur in regions with permafrost. Low temperatures and frozen conditions for much of the year slow soil-forming processes. 6. Histosols Organic soils with little or no climatic implications. Can be found in any climate where organic debris can accumulate to form a bog soil. Dark, partially decomposed organic material commonly referred to as peat. 7. Inceptisols Weakly developed young soils in which the beginning (inception) of profile development is evident. Most common in humid climates, they exist from the Arctic to the tropics. Native vegetation is most often forest. 8. Mollisols Dark, soft soils that have developed under grass vegetation, generally found in prairie areas. Humus-rich surface horizon that is rich in calcium and magnesium. Soil fertility is excellent. Also found in hardwood forests with significant earthworm activity. Climatic range is boreal or alpine to tropical. Dry seasons are normal. 9.Oxisols Soils that occur on old land surfaces unless parent materials were strongly weathered before they were deposited. Generally found in the tropics and subtropical regions. Rich in iron and aluminum oxides, Oxisols are heavily leached, hence are poor soils for agricultural activity. 10.Spodosols Soils found only in humid regions on sandy material. Common in northern coniferous forests and cool humid forests. Beneath the dark upper horizon of weathered organic material lies a light-colored horizon of leached material, the distinctive property of this soil. 11.Ultisols Soils that represent the products of long periods of weathering. Water percolating through the soil concentrates clay particles in the lower horizons (argillic horizons). Restricted to humid climates in the temperate regions and the tropics, where the growing season is long. Abundant water and a long frost-free period contribute to extensive leaching, hence poorer soil quality. 12. Vertisols Soils containing large amounts of clay, which shrink upon drying and swell with the addition of water. Found in subhumid to arid climates, provided that adequate supplies of water are available to saturate the soil after periods of drought. Soil expansion and contraction exert stresses on human structures

What are the 12 soil orders and their characteristics?

-Four basic soil structures are recognized: platy, prismatic, blocky, and spheroidal. Soil structure is important because it influences theease of a soil's cultivation as well as the susceptibility of a soil to erosion. In addition, soil structure affects the porosity and permeability of soil (that is, the ease with which water can penetrate). This in turn influences the movement of nutrients to plant roots. -Prismatic and blocky peds usually allow for moderate water infiltration, whereas platy and spheroidal structures are characterized by slower infiltration rate

What are the different types of soil structure and why is it important

Fall, slides, and flow

What are the three types of motion in mass movement?

-The most important of these are parent material, time, climate, plants and animals, and topography

What factors affect soil formation?

-If soil and regolith dominate, terms such as debris, mud, and earth are used. In contrast, when a mass of bedrock breaks loose and moves downslope, the term rock may be part of the description.

What terms are used to describe mass movements based on the material involved

-The optimal environment for chemical weathering is a combination of warm temperatures and abundant moisture. - In polar regions chemical weathering is ineffective because frigid temperatures keep the available moisture locked up as ice, whereas in arid regions there is insufficient moisture to foster rapid chemical weathering

What type of environment is the best for weathering?

topsoil

consists of the O and A horizons

gully erosion

removal of layers of soil, creating channels or ravines too large to be removed by normal tillage operations

immature soil

soil that lacks well-developed layers - are characteristic of new soils -are also characteristic of steep slopes, where erosion continually strips away the soil, preventing full development

residual soils

soils whose parent material is bedrock

mature soil

soils with clearley defined layers

angle of repose

the steepest angle at which loose material remains stationary without sliding downslope - depends on the size and shape of the particles - larger, more angular particles maintain the steepest slopes